Helios is a member of the Ikaros family of zinc finger transcription factors. It is expressed mainly in T cells, where it associates with Ikaros-containing complexes and has been proposed to act as a rate-limiting factor for Ikaros function. Overexpression of wild-type or dominant-negative Helios isoforms profoundly alters alphabeta T cell differentiation and activation, and endogenous Helios is expressed at strikingly high levels in regulatory T cells. Helios has also been implicated as a tumor suppressor in human T cell acute lymphoblastic leukemias. These studies suggest a central role for Helios in T cell development and homeostasis, but whether this protein is physiologically required in T cells is unclear. We report herein that inactivation of the Helios gene by homologous recombination does not impair the differentiation and effector cell function of alphabeta and gammadelta T cells, NKT cells, and regulatory T cells. These results suggest that Helios is not essential for T cells, and that its function can be compensated for by other members of the Ikaros family.